diff --git a/sanitizer/models.py b/sanitizer/models.py
index afc2590..229f27e 100644
--- a/sanitizer/models.py
+++ b/sanitizer/models.py
@@ -48,6 +48,8 @@ def to_python(self, value):
         return smart_unicode(value)
 
     def get_prep_value(self, value):
+        if value is None:
+            value = u''
         value = super(SanitizedTextField, self).get_prep_value(value)
         value = bleach.clean(value, tags=self._sanitizer_allowed_tags,
             attributes=self._sanitizer_allowed_attributes,